Beholder Mage

Absorption Cone. The Beholder Mage's central eye has been removed, however in its place the Beholder Mage has learned to absorb latent magical energies. The Beholder Mage creates an invisible 150-foot-cone in front of it, and if a creature in that area casts a spell or is the target of a spell, the Beholder Mage regains an expended spell slot equal to the spell level or lower. The area does not work against the Beholder Mage's own spells.

Autonomous Eyestalks. The Beholder Mage can concentrate on up to 3 spells at once. Each of these spells must be of a different level.

Legendary Resistance (3/day). If the Beholder Mage fails a saving throw, it can choose to succeed instead.

Spellcasting. The Beholder Mage is a 20th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 23, +15 to hit with spell attacks). The Beholder Mage knows the following spells in addition to Spells learned via its Spell Absorption, Knowledge Thief and expanded spell list features. It can cast them without material components:

Cantrips (at will): Fire Bolt, Sacred Flame, Thorn Whip, True Strike

1st level (4 slots): Absorb Elements, Expeditious Retreat, Guiding Bolt, Magic Missile, Shield

2nd level (4 slots): Blindness/Deafness, Earthbind,Invisibility, Misty Step, Scorching Ray, Spiritual Weapon

3rd level (4 slots): Aura of Vitality, Counterspell, Dispel Magic, Fireball, Lightning Bolt, Slow

4th level (3 slots): Blight, Death Ward, Dimension Door, Freedom of Movement

5th level (3 slots): Cone of Cold, Hold Monster, Telekinesis, Transmute Rock, Wall of Force, Wall of Stone

6th level (3 slots):Chain Lightning, Disintegrate, Eyebite

7th level (3 slots): Finger of Death, Plane Shift, Teleport

8th level (2 slots): Feeblemind, Mind Blank

9th level (2 slots): Power Word Kill, Wish

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 14 (4d6) piercing damage.

Spellcasting. The Beholder Mage casts a spell with a casting time of 1 action from its Spellcasting trait, expending a spell slot as normal.

Spell Rays. The beholder mage shoots three of the following magical spell rays at random (reroll duplicates). The spells are cast simultaneously, following the normal casting rules of the spell otherwise, including consuming spell slots. Spells chosen must have a casting time of 1 action and cannot be upcast. If the spell has a range of between 5 feet and 120 feet, the range is changed to 120 feet. If this action is used, the Beholder Mage cannot cast a spell as a bonus actions this turn.

1. Cantrip. The Beholder Mage casts a cantrip.
2. 1st Level. The beholder mage casts a 1st level spell. If it has no spell slots remaining, it instead casts Magic Missile at first level.
3. 2nd Level. The beholder mage casts a 2nd level spell. If it has no spell slots remaining, it instead casts Scorching Ray at second level.
4. 3rd Level. The beholder mage casts a 3rd level spell. If it has no spell slots remaining, it instead casts Lightning Bolt at third level.
5. 4th Level. The beholder mage casts a 4th level spell. If it has no spell slots remaining, it instead casts Blight at fourth level.
6. 5th Level. The beholder mage casts a 5th level spell. If it has no spell slots remaining, it instead casts Telekinesis at fifth level.
7. 6th Level. The beholder mage casts a 6th level spell. If it has no spell slots remaining, it instead casts Disintegrate at sixth level.
8. 7th Level. The beholder mage casts a 7th level spell. If it has no spell slots remaining, it instead casts Finger of Death at seventh level.
9. 8th Level. The beholder mage casts a 8th level spell. If it has no spell slots remaining, reroll this ray (there may not be duplicates).
10. 9th Level. The beholder mage casts a 9th level spell. If it has no spell slots remaining, reroll this ray (there may not be duplicates).

Spellcasting. The beholder mage casts a spell with a casting time of 1 bonus action from its Spellcasting trait, expending spell slots as normal, as long as it hasn't used Spell Rays this turn, or used spellcasting to cast a non-cantrip spell: Expeditious Retreat, Spiritual Weapon, Misty Step

The beholder mage can take up to three reactions per round but only one per turn.

Spell Absorption (Recharge 5-6). Immediately after a creature casts a spell that targets the beholder mage or includes it in an area of effect, the beholder mage can attempt to absorb the effects of that spell. The beholder mage must make an Intelligence (Arcana) check with the DC equaling 25+ the spells level.

On a success: the beholder mage learns that spell and gains a spell slot equal to the level of spell learned, which lasts until it is expended or the beholder mage takes a long rest. Additionally, the beholder mage automatically passes any relevant saving throws for that spell, and, and if a spell attack hits and deals damage, the beholder mage gains resistance to that instance of damage.

On a failure: the beholder mage has disadvantage on any relevant saving throws, and, if the spell requires an attack roll, it automatically turns into a critical hit (it still has to hit). If the beholder mage still has uses of Legendary Resistance, they may not be used against this spell.

Knowledge Thief. Immediately after witnessing a spell being cast, the beholder mage can attempt an Intelligence (Arcana)check with the DC equalling 20+ the spells level. On a success, the beholder mage learns that spell and can cast it using appropriate means.

Spellcasting. The beholder mage casts a spell with a casting time of 1 reaction from its Spellcasting trait, expending spell slots as normal: Absorb Elements, Shield, Counterspell

The beholder mage can take 3 legendary actions, choosing from the options below.. It can take only one legendary action at a time and only at the end of another creature’s turn. The beholder mage regains spent legendary actions at the start of its turn.

Spell Ray. The beholder mage uses its spell rays action, but only fires one ray.

Targeted Spell Ray (Costs 3 actions). The beholder mage casts a spell of its choice that it knows or has learned, ignoring casting time and spell components.